Electronic devices, such as pagers, cellular telephones, cordless telephones, and personal digital assistants (PDAs) just to name a few, have seen significant reduction in size due to the use of leadless components and improving reflow solder technologies. Such electronic devices typically utilize glass epoxy printed circuit boards upon which printed circuit patterns have been formed using conventional photolithographic processes and additive plating or etching processes. The printed circuit patterns are typically a one ounce copper plating, which after processing, is protected from oxidation using an organic solder preservative or tin-lead plating which has been leveled by an air-knife process. While both methods of protecting the copper from oxidation to guarantee subsequent solderability have generally proved satisfactory, the organic solder preservative is less costly to produce, but unfortunately provides little, if any post-solder protection, and the printed circuit board patterns of electronic assemblies often exhibit corrosion problems when subjected to moisture or humidity. Corrosion of the printed circuit pattern is especially troublesome at the connections of heat seal connectors such as used to interconnect liquid crystal displays to the copper printed circuit board runners.
The humidity problem can be resolved by the use of an electroless nickel-gold plating process, but such a plating process has proven to be less than satisfactory for widespread use due to embrittlement of component solder joints as compared to conventional reflow soldering of components to bare copper. As a result, the use of printed circuit boards plated using an electroless nickel-gold plating process has been limited to a very small percentage of the printed circuit boards manufactured. Joint embrittlement has been generally attributed to phosphorus buildup at the solder joint and the nature of the intermetallic formed at the joint during the reflow solder process. Such joint degradation has been limited to some extent by increasing solder pad sizes and the use of relatively high volumes of solder at the joint, both of which are contrary for the production of compact electronic circuits which are capable of tolerating high levels of mechanical shock.
What is needed is a solder for use in a soldering process which can overcome the joint embrittlement problem created in an electronic assembly which utilizes a nickel-gold finish.